1. Field of the Invention
The present invention relates to a microwave detector for detecting microwaves emitted from measuring devices and the like, and in particular relates to an improvement in the structure of the reception frequency conversion portion which carries out frequency mixing of the wave received by a horn antenna and the output of a local oscillator.
2. Description of the Prior Art
Microwave detectors which generate an alarm upon detecting microwaves having the specific microwave frequencies emitted by radar-type speed measurement devices are known in the prior art. In this connection, typical traffic monitoring radar-type speed measurement devices employ a plurality of microwave frequency bands, including the 10 GHz band (X band), 24 GHz band (K band) and 35 GHz band (Ka band). These kinds of speed measuring microwave detectors receive prescribed microwave bands with a super-heterodyne type circuit having a structure described below.
In general, a horn antenna is used for the reception antenna. Arranged at the feeding point of the slot portion of such antenna is a mixer diode. Further, a microwave circuit composed of a local oscillator is provided in the interior of the slot portion of the horn antenna. In such arrangement, the signals received by the horn antenna and the output of the local oscillator undergo frequency mixing in the mixer diode. Further, a suitable filter is provided between the mixing step and the local oscillator to prevent the reception input by the horn antenna from having an adverse effect on the operation of the local oscillator. In this regard, two general types of microwave detectors are known: a first type constructed from a local oscillator and a mixer, in which a microwave circuit uses a waveguide; and a second type constructed from a local oscillator and a mixing portion, in which a microwave integrated circuit uses a microstrip line.
In this connection, the present invention is directed toward the first type constructed from a microwave circuit which uses a waveguide, and an example of this type is disclosed in Japanese Laid-Open Patent Application No. HEI 5-308219. The invention disclosed in this publication is similar to that shown in FIG. 1. Namely, as shown in this drawing, a wide-area metallic film (e.g., copper foil) 11 having a prescribed pattern is formed on top of a printed substrate 10. Further, a microwave circuit component 12 made of die-cast aluminum is secured to the printed substrate 10 by screws so as to cover the metallic film 11 portion of the printed substrate 10. The microwave circuit component 12 includes a horn antenna portion 12a and a cavity portion 12b which together form an open space in the entire underside surface of the microwave circuit component 12, and this open space in the underside of the microwave circuit component 12 is filled by the metallic film 11 when the microwave circuit component 12 is secured to the top of the printed substrate 10. In this way, a horn antenna and a cavity which communicates with the back of a slot portion of the horn antenna are constructed by securing the microwave circuit component 12 to the top of the printed substrate 10. Further, screw-type adjustment pins 20a, 20b are provided near the horn antenna slot portion of the microwave circuit component 12.
A boss portion 12c is integrally formed in the slot portion of the horn antenna portion 12a of the microwave circuit component 12, and a through hole 13 which faces the boss portion 12c is formed in the printed substrate 10. Further, a pin-shaped leg 15a of a metallic pedestal 15 is fitted into the through hole 13 via a spring washer 16. The upper surface of the pedestal 15 includes a hole which is arranged coaxially with the hole of the boss portion 12c, and fitted into these holes of the boss portion 12c and pedestal 15 are the pin portions of both ends of a bill-type mixer diode 14. In this way, the mixer diode 14 is arranged at the slot portion feeding point of the horn antenna in a sandwiched state between the printed substrate 10 and the microwave circuit component 12.
Further, a microwave substrate 17 is housed inside the cavity enclosed between the metallic film 11 of the printed substrate 10 and the cavity portion 12b of the microwave circuit component 12. A local oscillator is mounted in the microwave substrate 17. The oscillatory output of the local oscillator is emitted into the space inside the cavity and reaches the slot portion of the horn antenna, and as described above, because the mixer diode 14 is arranged at the slot portion, frequency mixing of the antenna input and the first local oscillator output are carried out in a microwave circuit mode.
Further, although omitted from the drawings, an intermediate frequency processing circuit portion is horizontally mounted at a peripheral portion of the through hole on the underside of the printed substrate 10. This intermediate frequency processing portion includes a first intermediate frequency filter for inputting the output of the mixer diode 14, a second local oscillator, and a second mixer circuit for carrying out frequency mixing of the output of the second local oscillator and the output of the first intermediate frequency filter. Also, a sealed case is secured to the underside of the printed substrate 10 to cover this intermediate frequency processing circuit portion.
As for the pattern of the metallic film 11 on top of the printed substrate 10, it is designed not only to fill the space underneath the microwave circuit component 12, but to function as a shield for the intermediate frequency processing circuit portion as well. Further, low frequency circuits such as a wave detection circuit, a reception signal discrimination circuit and an alarm circuit are mounted on another region of the printed substrate 10.
Now, as shown in FIG. 2, the mixer diode 14 is constructed from a cylindrical ceramic tube 14a which is open at both ends, and an anode electrode 14b and a cathode electrode 14c which are mounted to the ceramic tube 14a to plug both ends thereof. Further, a diode chip 14d is mounted to the upper end of the cathode electrode 14c (inside the ceramic tube 14a), and this diode chip 14d is electrically connected to the anode electrode 14b by bonding wires 14e.
Unfortunately, because there is no amplification circuit in the circuit design described above in which the reception signals and output of the local oscillator undergo frequency mixing in the mixer diode 14 and are directly converted to an intermediate frequency, the frequency conversion loss of the mixer circuit has a large effect on sensitivity. Accordingly, in order to improve the sensitivity, it is essential to make such conversion loss as small as possible. In this regard, the conversion loss is related to many factors, such as the characteristics of the mixer diode, the input voltage from the local oscillator, the impedance matching between the mixer diode and the antenna, and the impedance matching between the mixer diode and the local oscillator.
In the bill-type structure described above, the main factors which determine the impedance of the mixer diode 14 are, in addition to the junction capacitance and series resistance inside the diode, the package capacitance and the inductance of the bonding wire 14e. Further, the higher the frequency of the microwave and milliwave, the larger the effect of such factors.
Further, a Ga-As Schottky barrier-type diode is generally used for the microwave mixer diode 14. The junction capacitance inside the diode is semiconductor characteristic and is determined by the semiconductor material and the structure of the device. In the same way, the package capacitance and the bonding wire inductance are determined by the package material and structure. However, even though conversion loss can be lowered by lowering the impedance, it is difficult to remove the inherent impedance component of the circuit component described above.